1. Field of the Invention
The present invention relates to an anode comprising an anode current collector and an anode active material layer, and a battery using the anode.
2. Description of the Related Art
In recent years, as mobile devices have higher performance and more functions, higher capacities of secondary batteries as power sources of the mobile devices have been desired. As a secondary battery which meets the requirement, a lithium secondary battery is cited. However, the battery capacity of a currently typical lithium secondary battery which uses lithium cobalt oxide as a cathode and graphite as an anode has reached a point of saturation, so it is extremely difficult to substantially increase the capacity of the lithium secondary battery. Although an anode using lithium (Li) metal has been studied since a long time ago, in order to put the anode to practical use, it is required to improve lithium precipitation/dissolution efficiency and control dendritic precipitation.
On the other hand, a secondary battery using an anode with a high capacity which uses silicon (Si), germanium (Ge), tin (Sn) or the like has been actively studied recently. However, when charge and discharge are repeated, the anode with a high capacity is broken into small pieces due to severe expansion and shrinkage of an active material, thereby a current collecting property declines, or the decomposition of an electrolyte solution is accelerated due to an increase in a surface area, thereby cycle characteristics are extremely poor. Therefore, when an anode formed through forming an active material layer on a current collector by a vapor-phase deposition method, a liquid-phase deposition method, a sintering method or the like is used, the anode can be prevented from being broken into small pieces, compared to a conventional coating type anode formed through applying slurry including a particulate active material, a binder and the like to a current collector, and the current collector and the active material layer can be formed as one unit. Therefore, the electronic conductivity in the anode is extremely superior, and higher performance in terms of capacity and cycle lifespan is expected. Moreover, an electronic conductor, a binder and voids which are present in a conventional anode can be reduced or eliminated, so the anode can be formed into a thin film in essence.
However, even in the anode, the cycle characteristics are not sufficient because of a nonreversible reaction of an active material according to charge and discharge. Moreover, as in the case of a conventional anode with a high capacity, the reactivity with an electrolyte is still high, so the capacity largely declines due to a reaction with the electrolyte according to charge and discharge especially in the early stages of cycles. Further, in the anode with a high capacity, an anode potential is largely increased according to extraction of lithium especially in the late stages of discharge, which is one of factors causing a decline in characteristics.
In order to overcome the problems, a method of inserting lithium related to a battery reaction into the anode in advance is considered. For example, an anode in which lithium is inserted into an anode material made of silicon or germanium with an ion injector in advance (refer to Japanese Unexamined Patent Application Publication No. 2002-93411), and a battery in which a cathode and an anode are formed in a state where alkali metal ions can be inserted, and the cathode and the anode are brought into contact with a dispersant formed through dispersing an alkali metal in an organic solvent including a compound capable of being solvated or forming a complex with alkali metal ions to insert the alkali metal into the battery (refer to Japanese Unexamined Patent Application Publication No. Hei 11-219724) have been reported. Moreover, in a conventional lithium-ion secondary battery using carbon for an anode, a large number of techniques of inserting a predetermined amount of lithium into the anode in advance have been reported. For example, an anode using particles with a structure in which lithium metal layer and a carbon layer are alternately laminated (refer to Japanese Unexamined Patent Application Publication No. Hei 7-326345), an anode in which a thin film made of transition metal chalcogen compound or a carbon material electrochemically supports an alkali metal (refer to Japanese Patent No. 3255670), an anode in which lithium metal foil is affixed to disperse and retain lithium in a carbon material (refer to Japanese Patent No. 3063320), an anode in which lithium is introduced through injecting an electrolyte solution to establish a short circuit between lithium metal and a carbon material (refer to Japanese Unexamined Patent Application Publication No. Hei 10-270090), a lithium secondary battery in which aromatic hydrocarbon forming a complex with a lithium metal is added to an anode in which a short circuit is established between a carbon material and lithium metal (refer to Japanese Unexamined Patent Application Publication No. Hei 11-185809) and a lithium secondary battery comprising a supply member made of a lithium metal housed in a battery case so as not to electrically make contact with an anode (refer to Japanese Unexamined Patent Application Publication No. 2001-297797) have been reported.
However, the anode into which lithium is inserted in advance is in a charge state, so the anode is in a active state, thereby the surface of the anode is oxidized to form a coating of lithium oxide or lithium hydroxide. The coating causes an increase in impedance in the battery and degradation in battery characteristics. Moreover, depending upon the thickness of the formed coating, the impedance in the battery differs, so even in batteries with the same structure, the thickness of the formed coating differs depending upon a handling environment and storage conditions, so variations in characteristics between each battery occurs.